1. Field of the Invention
The present invention is directed to a method and apparatus for obtaining an ultrasound image of an examination subject having image-disturbing inhomogeneities, and in particular to a method and apparatus wherein the disturbances caused by the inhomogeneities are substantially compensated so that the resulting image is free of artifacts.
2. Description Of The Prior Art
Attempts have been made in B-image scanners to achieve a high lateral resolution by employing applicators (ultrasound arrays) having extremely large active apertures (usually in reception). This goal has been achieved in the case of homogeneous phantoms and in some of the examined patients. The clarity and resolution of ultrasound B-images from living tissue, however, is frequently noticibly less than that from phantoms A not negligibly low percentage of patients is so inhomogeneous in sound-physical terms that no gains can be acquired from the large aperture. The corresponding ultrasound images are covered by a "gray veil." This is attributed to the topical dependency of the speed of sound in the tissue, particularly in the subcutaneous fatty tissue which causes refractive effects
Specifically in the field of high-quality B-scanners having extremely good resolution, which can practically be obtained only given employment of large apertures, it would be of significance if the percentage of patients that can be scanned with high image quality could be successfully increased. A method and a means would thus be desirable which reduce or largely eliminate the influence of, in particular, the inhomogeneities of the examined patient close to the skin in a medical ultrasound imaging apparatus given B-image acquisition. The functional principle must be capable of being applied without serious disadvantages for patient and examining personnel. It would be acceptable if the image were offered not in real time, but only as a still picture, or if a real time image were only supplied after a waiting time and restricted to one application location.
A method and imaging portrail system for examining soft tissue are disclosed by PCT publications WO No. 84/01433 (U.S. Pat. No. 4,484,477), WO No. 84/01434 (U.S. Pat. No. 4,471,785) and WO No. 84/01435. In accord therewith, disturbances in the image quality are reduced by correcting the delay times and amplitude occupancy of the active aperture of the ultrasound array. The correction is preferably carried out on the basis of a cross-correlation of signals of the elemental or primary transducer elements of the array, with more details regarding the precise procedure not being provided in the above documents. Value is attached to high processing speed, requiring special calculating circuits and thus an extremely high outlay. A cost-beneficial embodiment would be desirable.